1. Field of the Invention
The present invention relates to a resonant-type switching power supply device and more particularly to the resonant-type switching power supply device made up of switching power supply circuits connected in parallel.
2. Description of Related Art
FIG. 9 shows one example of a conventional half-wave rectification resonant-type switching power supply device. The half-wave rectification resonant-type switching power supply device has switching elements 2 and 3 connected in series between a positive terminal and a negative terminal of a dc power source Vi and a transformer 4 connected between a connecting point of the switching elements 2 and 3 and the negative terminal of the dc power source Vi.
Moreover, between a connecting point of the switching elements 2 and 3 and a cold side terminal of a primary winding 4a of the transformer 4 is connected a reactor 5. Between a hot side terminal of the primary winding 4a of the transformer 4 and a negative terminal of the dc power source Vi is connected a current resonance capacitor 6. That is, a serial circuit made up of the reactor 5, primary winding 4a, and serially connected current resonance capacitor 6 is connected in parallel across the switching element 3. Across the switching element 3 is serially connected a voltage resonance capacitor 7.
To a hot side of the secondary winding 4b of the transformer 4 is connected an anode of a diode 8 constituting a rectifying circuit and a cathode of the diode 8 is connected to an anode of a smoothing capacitor 9. The above resonant-type switching power supply device is so configured that a direct output voltage is fed to a load, for example, a resistor Ro from an anode of the smoothing capacitor 9.
In the above resonant-type switching power supply device, the switching elements 2 and 3 are alternately on/off controlled. While the switching element 2 is on and the switching element 3 is turned off, resonance occurs among the reactor 5, exciting inductor Lp of the primary winding 4a, and current resonance capacitor 6. At this time point, a resonance current flows from the positive terminal of the direct current power source Vi to the reactor 5, primary winding 4a, current resonance capacitor 6. As a result, the current resonance capacitor 6 is charged. Also, when the switching element 2 is turned off and while the switching element 3 is turned on, a charging voltage of the current resonance capacitor 6 is applied to the primary winding 4a of the transformer 4, when a voltage across the primary winding 4a is inverted and, as a result, a diode 8 connected to the secondary winding 4a of the transformer 4 is turned on.
Therefore, resonance occurs in a circuit constituted of the reactor 5 and current resonance capacitor 6. The resonance current is reduced due to discharge of the current resonance capacitor 6. The resonance current then flows in a reverse direction and transfers power energy to the secondary winding 4b of the transformer 4. A current by the power energy transferred to the secondary winding 4b is rectified via the diode 8. The rectified current is fed to the smoothing capacitor 9 for charging. The smoothing capacitor 9 supplies direct current power to the resistor Ro. Moreover, the switching elements 2 and 3 are controlled so as not be simultaneously turned on. That is, the switching elements 2 and 3 are alternately turned on or off with dead time. The voltage resonance capacitor 7 produces voltage resonance when the switching elements 2 and 3 are turned on or off.
Power energy transferred to the secondary side of the transformer 4 is determined depending on charge capacity of the current resonance capacitor 6. Therefore, by changing the period during which the switching element 2 is on, the power energy to be transferred to the secondary side of the transformer 4 can be changed. Also, the power energy to be transferred to the secondary side of the transformer 4 corresponds to a resonance current caused by the current resonance capacitor 6 and the reactor 5. The period during which the power energy is transferred to the secondary side is constant which does not depend on a length of the period during which the switching element 3 is on. The control of the switching element to control power energy to be transferred to the secondary side includes PWM (pulse-width modulation) control in which a switching frequency is made constant and the period during which the switching element 2 is on is made variable or frequency control in which the period during which the switching element 2 is on is made variable and the period during which the switching element 3 is on is made constant.
In general, the miniaturization of the switching power supply device is made possible by making high a frequency for off-operation. In the conventional half-wave rectification resonant-type switching power supply device as shown in FIG. 9, in order to make high a switching frequency of the switching elements 2 and 3, it is necessary to increase a resonance frequency which is determined by the reactor 5 and current resonance capacitor 6. Generally, to reduce component counts of a device, instead of the reactor 5, a leakage inductance of the transformer 4 may be used. It is difficult to finely adjust inductance of the reactor 5 and, therefore, resonance frequency is adjusted by changing capacity of the current resonance capacitor 6. However, if the capacitor of the current resonance capacitor 6 is made small, a current that can be flown through the current resonance capacitor 6 becomes smaller, which causes the difficulty in transferring of great power energy to the secondary side.
Conventionally, to solve this problem, in parallel-connection of the switching elements of the switching power supply device to take out great power energy as disclosed in Unexamined Japanese Patent Application Publication No. H10-229676 is envisioned.
However, the configurations in which a plurality of the conventional half-wave rectification resonant-type switching power supply devices shown in FIG. 9 are connected in parallel and the smoothing capacitor 9 is commonly used by the devices presents a problem in the power energy fed from the resonant-type switching power supply devices to the smoothing capacitor 9 can not be equal resultant in a low efficiency of the power supply device.